If those "microwave weapons" end up enabling the killing of millions of people (like e.g. nuclear weapons can wipe out the whole earth), then those left will be right to say:
"Hmm, maybe we were better off without microwave technology after all. In fact, just because we can make something, probably doesn't mean that we should make it".
Sure, it's always a concern. Just because we make something doesn't mean that we should use it. So you've already got that ethical dilemma covered on the basis of usage, if not invention.
On the other hand, if you don't develop technology, you'll never know if it will be used for good, and you'll never see the advantages if it is. And failing to develop something does nothing to prevent someone else from developing it later. If something is possible, you want to be the one making it possible, not the one cowering in fear while someone else forges on ahead.
If those "microwave weapons" end up enabling the killing of millions of people (like e.g. nuclear weapons can wipe out the whole earth), then those left will be right to say:
"Hmm, maybe we were better off without microwave technology after all. In fact, just because we can make something, probably doesn't mean that we should make it".